In general, the present invention concerns improvements in the manufacturing of capacitors which are formed by plural windings of metalized thermoplastic sheets or elongated films. These devices are often referred to as metalized thermoplastic capacitors.
The method of manufacturing such capacitors is generally set forth in the present inventor's copending application Ser. No. 487,249 filed July 10, 1974. Briefly, metalized thermoplastic capacitors are formed by first applying a coating of good conductive material or binding a sheet of conductive material to one surface of each of two elongated stretches of thermoplastic sheet or film material. An uncoated margin is provided on opposite edges of each sheet so that the dielectric film insulates the conductive coating from direct contact when the sheets are wound together in bifilar configuration. Each axial end of the wound body has the conductive surface of one sheet exposed, and electrodes are attached to the exposed conductive surfaces by means of a molten metal spray. The method also applies to other types of capacitors, and the specification uses the capacitor of Ser. No. 487,249, supra, as an example.
In the old method, the two layers of thermoplastic dielectric are wound on a mandrel to a given degree of tightness. The wound part is then subjected to heat treatment, generally employing a vacuum, to shrink the thermoplastic and tighten the winding. The shrinkage imparts physical rigidity as well as capacitive stability to the wound part.
It is well known that moisture or other foreign material effects the electrical properties of a capacitor. Heretofore, every effort has been made to eliminate the foreign material from the capacitor body at an early stage in the manufacturing process. For example, see Cox, U. S. Pat. Re 27,533.
It is advantageous to obtain a very tight winding from the physical stability standpoint of the capacitor, and the tightness of the winding also adds capacitance stability to the capacitor. However, tight winding and heat shrinking to obtain an even tighter winding may cause physical problems with the material of the capacitor. During both the winding and heat shrinking, successive layers of material abrade each other which may cause cracking of the capacitor, splitting of an internal layer, or removal from an area of the conductive coating. Any of these defects yield undesirable physical and electrical properties in the capacitor.
It is also an object of the present invention to increase the insulation resistance of the capacitor. A higher insulation resistance causes a capacitor to be closer to the ideal capacitor and improves the RC response of a circuit utilizing it.
Therefore, it is an object of the present invention to provide a method in which the capacitor is tightly wound and heat shrunk without adverse affects to the material and the capacitor. A further object involves accomplishing the previous objective without adversely effecting the electrical properties of the capacitor.